Organic El Display Device

ABSTRACT

[Problem]To provide an organic EL display device which is suitable to minimize a size thereof and is capable of preventing erroneous light emission and reducing power consumption when a display is switched from one of a first and second organic EL display panels to the other. To provide an organic EL display device capable of preventing erroneous light emission with which display luminance in black level is pushed up to that in gray level. [Means for Solving the Problem]In the first invention, the current drive circuit having output pins common for the first and second organic EL panels is provided. Therefore, there is no need of providing current drive circuits for the respective first and second organic EL panels. Consequently, it is unnecessary to make the organic EL panel, which is not selected, standby state, so that it is possible to reduce power consumption correspondingly. In the second invention, the first and second organic EL panels of the organic EL display device are of the passive matrix type and first diodes for preventing reverse current flow are provided between column lines of the organic EL panels and terminal pins of the organic EL panels.

TECHNICAL FIELD

This invention relates to an organic EL display device and, inparticular, to an organic EL display device, which has a main displaypanel and a sub-display panel and is capable of reducing powerconsumption when a display is switched from one of the display panels tothe other and is suitable to minimize the size and the thicknessthereof.

BACKGROUND ART

The organic EL display device can perform high luminance display due tospontaneous light emission and is suitable for a small area display.Therefore, the organic EL display device has been paid attention as anext generation display device to be mounted on a portable telephoneset, a PHS, a DVD player or a PDA (digital portable terminal device).

In a portable telephone set, for example, a main display panel and asub-display panel are arranged back to back. A switching of display isperformed between the sub-display panel provided on a front side of acover of the organic EL display device to display information necessaryfor the sub-display panel when the cover is closed and to displayoperation information such as a menu on the main display panel providedon a rear side of the cover when the cover is opened.

In this case, the main display panel is for a high resolution colordisplay and the sub-display panel having display area smaller than thatof the main display is for a monochromatic display. In particular, thesub-display panel of the portable telephone set displays time and imageof a call when a signal is received.

Drivers of the main display panel and the sub-display panel havedifferent specifications and are chipped on respective displaysubstrates. Therefore, these drivers are usually provided separately.

A current drive circuit of the organic EL display panel includes currentsource drive circuits, for example, output circuits formed by currentmirror circuits corresponding to respective terminal pins (column pins),regardless of the type of the organic EL panel, active matrix type orpassive matrix type.

In the active matrix type organic EL panel, pixel circuits are providedcorrespondingly to display cells (pixels) and each pixel circuit drivesa transistor according to a voltage stored in a capacitor and theorganic EL element (referred to as “OEL element”, hereinafter) iscurrent-driven by the transistor.

On the other hand, in the passive matrix type organic EL panel, anodesof the OEL elements arranged in a matrix are connected to output pins ofdrive current sources through column pins and the OEL elements aredriven by the respective drive circuits.

Incidentally, a drive circuit for an organic EL display panel, which isconstructed with D/A converter circuits provided correspondingly tocolumn pins, is disclosed in JP2003-234655A (Patent Reference 1). In thedrive circuit disclosed in Patent Reference 1, the D/A convertercircuits provided correspondingly to the column pins receive displaydata and a reference drive current, converts the digital display datainto an analog signal according to the reference drive current andgenerates drive currents corresponding to the respective column pins ora current on which the drive currents are generated.

Patent Reference 1: JP2003-234655A

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

In the main display panel and the sub-display panel, driver ICs havingcurrent source drive circuits provided correspondingly to data lines orcolumn pins are provided, respectively. Therefore, an area of a compactelectronic device such a portable telephone set for mounting the maindisplay panel and the sub-display panel becomes large, so that, forexample, reduction of thickness of a cover side casing of the compactelectronic device is difficult.

When one of the main display panel and the sub-display panel isoperated, a drive current source of the other display panel is notcompletely cut off and the other display panel becomes a standby state.Therefore, power consumption is increased correspondingly. When aswitching of display between the main display panel and the sub-displaypanel is performed, the standby setting of the drive circuit of one ofthe main display panel and the sub-display panel is performed and arecovery from the standby state of the drive circuit of the otherdisplay panel is performed. Therefore, power consumption is increased bytransient current due to the switching.

In order to restrict the increase of power consumption, it may beconsidered to use a driver IC for the main display panel and thesub-display panel commonly. In such case, it is considered that thenumber of output pins of the drive IC connected to respective columnpins is doubled and the output pins are switched within the driver IC.However, such solution is very difficult due to increase of the numberof output pins. In addition, when switches are provided correspondinglyto the respective output pins, the circuit size becomes very large.

In particular, when organic EL display panels of the passive matrixtype, which requires large output current, are used for both the maindisplay panel and the sub-display panel, an OEL element of the maindisplay panel and an OEL element of the sub-display panel are connectedto one output pin in parallel through respective column pins, ascapacitive loads. Therefore, there is a problem of erroneous emission ofOEL elements of the display panel, whose display is terminated, due totransient phenomenon before and after the switching.

The present invention was made to solve the above mentioned problems ofthe prior art and an object of the present invention is to provide anorganic EL display device, which is possible to prevent erroneous lightemission of display panels when a display is switched from one displaypanel to the other, to reduce power consumption of the organic ELdisplay panel when the display is switched and to make the organic ELdisplay device suitable to reduce size and thickness thereof.

Incidentally, when the main display panel and the sub-display panel areof the passive matrix type and are driven by a common driver, columnlines of the display panel on the display drive side (lighting side) arealso connected to column lines of the other display panel on the displaytermination side (non-lighting side) through output pins of the driver.

The OEL element is a capacitive element and a number of OEL elements areconnected to the column lines on the non-lighting side. Therefore, whenit is driven by the common driver, the column lines of the lighting sideare connected to other column lines on the lighting side throughparasitic capacitance of the OEL elements on the non-lighting side.Further, in the passive matrix type organic EL panel in which a numberof OEL elements are connected to column lines, parasitic capacitancelooked from a certain column line becomes large, so that there is aproblem that a drive current from the other driven column line flowsindirectly to a specific column line through the parasitic capacitance.

Particularly, in a high resolution QVGA full color display, the numberof pins of each primary color is 120 and totally 360 pins are necessary,so that three drivers are necessary presently. Therefore, the number ofOEL elements connected to one column driver IC is (number of columnpins)×(number of row side pins) and, in the main display panel, itbecomes 10,000 or more and, in the sub-display panel, it becomes 5,000or more. Although capacitance of each OEL element is as small as severalpF, capacitance increases with increase of the number of OEL elements,so that drive current flowing from one column line to another isincreased.

It has been found that, when a certain column line of the display panelon the lighting side becomes black display (drive current “0”), there isa problem of erroneous emission that the black level display is pushedup to gray level due to the indirect flow of the drive current.

Another object of the present invention is to provide an organic ELdisplay device capable of preventing erroneous emission by pushing blacklevel up to gray level when a certain column line of the display panelon the lighting side becomes black display.

Means for Solving the Problem

In order to achieve the objects of the first present invention, anorganic EL display device, which includes a first and second organic ELpanels, selects either one of the organic EL panels according to aselection signal and drives the selected organic EL panel to perform apredetermined display, comprises a plurality of current drive circuitshaving output pins connected commonly to data lines or column pins ofthe first and second organic EL panels, for outputting drive currents ofOEL elements from the output pins to the data lines or the column pinsconnected to the output pins, a reset circuit connected to the outputpins for resetting terminal voltages of the OEL elements to apredetermined voltage in a reset period and a first and second scancircuits provided correspondingly to the first and second organic ELpanels for scanning scan lines in a row direction or a verticaldirection of the first and second organic EL panels, wherein one of thescan circuits corresponding to one of the organic EL panels, which is tobe driven, is operated in the reset period according to the selectionsignal and an operation of the other scan circuit is stopped or the scanoperation of the other scan circuit itself is stopped for driving saidone organic EL panel and ceasing the other organic EL panel.

According to second invention of the present invention, the first andsecond organic EL panels of the organic EL display device are of thepassive matrix type and first diodes for preventing reverse current floware provided between column lines of the organic EL panels and terminalpins of the organic EL panels.

ADVANTAGE OF THE INVENTION

In the first invention, the current drive circuit having output pinscommon for the first and second organic EL panels is provided.Therefore, there is no need of providing current drive circuits for therespective first and second organic EL panels. Consequently, it isunnecessary to make the organic EL panel, which is not selected, standbystate, so that it is possible to reduce power consumptioncorrespondingly.

Further, since the display switching is performed by selecting the scancircuit for scanning the scan line in the row direction of the verticaldirection, there is no need of providing a plurality of switchescorrespondingly to the output pins to thereby restrict an increase ofthe circuit size. Further, since the display switching from one of theorganic EL panels, which is displaying, to the other is performed by thereset control pulse, erroneous emission of the other organic EL panelcan be prevented. Further, when the driving of the displaying organic ELpanel is ended by making impedance of the output terminals connected tothe row side scan line (one horizontal scan line in the verticaldirection) of the scan circuit of the displaying organic EL panel highimpedance (Hi-Z), it is possible to restrict increase of loadcapacitance of the non-displaying organic EL panel, which is parallelload of the displaying organic EL panel to thereby restrict increase ofpower consumption.

In the second invention, since the first diodes for preventing reversecurrent flow are provided between the column lines and the terminal pinsof the organic EL panels, parasitic capacitance due to the first diodesis inserted in parallel to the column lines of the non-displayingorganic EL panel. Therefore, the drive current flowing from other columnlines of the displaying organic EL panel to a certain column pin passesthrough the parasitic capacitance of the first diode. Since theparasitic capacitance of the first diode is very small, it is possibleto restrict the drive current flowing through the first diode to a smallvalue. In addition, it is possible to prevent the reverse flow of thedrive current by the first diode. Consequently, in the displaying panel,there is substantially no pushing up of the column lines displayingblack level to gray level. Even when there is the pushing-up, the levelpushed up is small and it is possible to prevent erroneous emission ofblack level, which may be recognized by eyes.

Consequently, it is possible to prevent the erroneous emission andreduce power consumption when the display switching between the maindisplay panel and the sub-display panel is performed, to thereby realizea compact and thin organic EL display device.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a block circuit diagram according to an embodiment of theorganic EL display device of the present invention in a case where acurrent drive circuit is used for two passive matrix type organic ELpanels, FIG. 2 is a timing chart when a display switching is performed,FIG. 3 illustrates an organic EL panel, a display of which is stoppedwhen a display switching of a row side scanning circuit is performed,FIG. 4 shows an embodiment capable of preventing erroneous lightemission due to pushing up of display luminance of the column lines onthe lighting side display panel from black level to gray level and FIG.5 is a circuit for explaining load impedance of the non-lighting sidecolumn line in the embodiment shown in FIG. 4.

In FIG. 1, an organic EL display device 1 includes passive matrix typeorganic EL panels 2 and 3.

A drive IC (referred to as “IC”, hereinafter) 4 provided commonly forthe organic EL panels 2 and 3 includes column side output stage currentsources 40 a, . . . 40 i, . . . 40 n, row side scanning circuits 41 and42, an inverter 43 and a reset circuit 44.

The reset circuit 44 is composed of analog switches (transmission gates)44 a, . . . 44 i, . . . 44 n connected to respective output pins 5 a, .. . 5 i, . . . 5 n and a constant voltage diode Dz.

The driver 4 drives one of the organic EL panel 2 and the organic ELpanel 3 according to a display selection signal (referred to as“selection signal SEL”, hereinafter) “H” (HIGH level) or “L” (LOW level)supplied from a control circuit 12 through an input terminal 4 a to makeone of the organic EL panels 2 and 3 displayable and the other notdisplayable.

The control circuit 12 generates the selection signal SEL “H” when adisplay switch 11 is turned, for example, ON. When the display switch 11is in OFF state or the display switch 11 is turned OFF, the controlcircuit 12 sets the selection signal SEL in “L” (LOW level) and outputsthe selection signal SEL “L”.

Incidentally, the display switching switch 11 in, for example, a potabletelephone set having the display device 1, is activated and turned ONwhen a cover of the telephone set is closed.

The control circuit 12 includes a one-shot circuit 12 a, an OR circuit12 b, a timing signal generator circuit 12 c and a selection signalgenerator circuit 12 d and generates the selection signal SEL “H” or “L”correspondingly to the ON/OFF operation of the display switch 11 causedby opening and closing of the cover of the potable telephone set.

The organic EL panel 2 is provided as a main display panel on a rearsurface side of the cover of the portable telephone set housing thedisplay device 1 and the organic EL panel 3 is provided as a sub-displaypanel on a front surface side of the cover of the portable telephoneset. These organic EL panels 2 and 3 are housed back to back in a coverside casing of the device and the driver 4 is arranged between theorganic EL panels 2 and 3 and has output pins connected to column lines(column pins) of the organic EL panels 2 and 3, respectively.

Incidentally, when the organic EL panels 2 and 3 are arranged back toback as mentioned and driven, a horizontal scan direction of one of theorganic EL panels is opposite to a horizontal scan direction of theother organic EL panel. Therefore, it is necessary that display data for1 line in horizontal scan direction of one of the organic EL panels 2and 3 is set in reverse direction with respect to the other. In suchcase, it is usual to use a bidirectional shift register. Since suchmatter is not directly related to the present invention, detaileddescription thereof is omitted.

The number of display pixels of the organic EL panel 2 (main displaypanel) is usually different from that of the organic EL panel 3(sub-display panel). For example, the number of pixels of the organic ELpanel 2 is 160 column lines×128 row lines and the number of pixels ofthe organic EL panel 3 is 96×96.

In the following description, however, 96 output pins of the organic ELpanel 2, which are common for the organic EL panel 3, will be described.Incidentally, as to 97 to 160 output pins of the organic EL panel 2,which are not common for the organic EL panel 3, there is no problemsince there is no output currents generated at the 97 to 160 output pinsby making display data to be set in D/A converters 46 (refer to FIG. 1)corresponding to the output pins, which are 0 when a display is ceasedby the display switching. Therefore, the connection of the 97 to 160output pins is not shown in FIG. 1.

Column lines Xa, . . . Xi, . . . Xn of the organic EL panel 2corresponding to 1 horizontal line and column lines Xa, . . . Xi, . . .Xn of the organic EL panel 3 are connected to the respective output pins5 (representing output pins 5 a, . . . 5 i, . . . 5 n) of the outputstage current sources 40 (representing the output stage current sources40 a, . . . 40 i, . . . 40 n) through the respective column pins.

Each of the row side scan circuits 41 and 42 is constructed mainly withshift registers and CMOS output circuits 6 (refer to FIG. 3).Incidentally, in FIG. 3, each CMOS output circuit 6 is shown as a switchcircuit including a pair of switches. One of the switches of the switchcircuit connected between a power source line +Vcc and a cathodeconnection line Y (Y1, Y2, Y3, . . . Yi . . . ) of the OEL elements isconstructed with a P channel MOS transistor and the other switchconnected between the cathode connection line Y (Y1, Y2, Y3, . . . Yi .. . ) of the OEL elements and ground GND is constructed with a N channelMOS transistor.

As shown in FIG. 1, the output stage current source 40 is constructedwith a current mirror circuit 45 and the D/A converter circuit 46. Thecurrent mirror circuit 45 is constructed with P channel MOS transistorsQP1 and QP2. Channel width (gate width) ratio of the input sidetransistor QP1 and the output transistor QP2 is 1:10.

Sources of the transistors QP1 and QP2 are connected to the power sourceline +Vcc of about +15V. A drain of the input side transistor QP1 isconnected to a common gate and to an output of the D/A converter 46.

The D/A converter 46 converts a reference drive current into an analogcurrent corresponding to display data inputted to the input sidetransistor of the current mirror circuit.

Each of the analog switches 44X (representing analog switches 44 a, . .. 44 i, . . . 44 n) is in ON state during a reset period RT determinedby a reset signal RS inputted from the control circuit 12 through theinput terminal 4 b. As shown in FIG. 2(c) and FIG. 2(g), the resetsignal RS is “H” during the reset period RT and is usually generatedcorrespondingly to a reset control signal (or a timing control signalRSc). Therefore, the respective output pins 5 are set to voltage VR of aconstant voltage diode Dz during the reset period to perform a constantvoltage resetting (presetting).

Each of the row side scan circuits 41 and 42 performs the scanningoperation in response to an enable signal and the reset control signalRSc, which are in “H” state. The reset control signal RSc is suppliedfrom the timing signal generator circuit 12 c of the control circuit 12through an input terminal 4 c.

The row side scan circuit 41 receives a selection signal SEL through theinput terminal 4 a and the inverter 43 as the enable signal. The rowside scan circuit 42 directly receives the selection signal SEL as anenable signal.

Incidentally, the scanning operations of the row side scan circuits 41and 42 are started at a start of the reset period RT of the resetcontrol signal RSc.

When the display switch 11 is turned OFF from ON, the row side scancircuit 41 receives the selection signal SEL “L” through the inverter 43as the enable signal and starts the scanning operation in verticaldirection (row side) for the organic EL panel 2 in the reset period RTof the reset control signal RSc. On the other hand, when the displayswitching switch 11 is turned OFF, the row side scan circuit 42 of theorganic EL panel 3 receives the selection signal SEL “L” directly andterminates the scanning operation in the vertical direction.

On the other hand, when the row side scan circuits 41 and 42 receive theenable signal “L”, the two switches of each CMOS output circuit 6, whichare constituted with the P channel MOS transistor and the N channel MOStransistor, respectively, are turned OFF as shown in FIG. 3 to set anoutput of high impedance (Hi-Z).

On the contrary, when the display switching switch 11 is turned ON fromOFF, the row side scan circuit 41 of the organic EL panel 2 receives theselection signal SEL “H” through the inverter 43 as the enable signal,so that the scanning operation in the vertical direction is terminated.On the other hand, when the display switch 11 is turned ON, the row sidescan circuit 42 of the organic EL panel 3 receives the selection signalSEL “H” as the enable signal, so that the scan operation in the verticaldirection is started from the rising edge of the reset period RT of thereset control signal RSc.

As such, in, for example, the portable telephone set having the organicEL display device 1, the selection signal SEL becomes H when the coverof the portable telephone set is closed, so that the row side scancircuit 42 of the organic EL panel 3 operates and the selection signalSEL becomes “L” when the cover is opened, so that the row side scancircuit 41 of the organic EL panel 2 operates.

Further, the organic EL panel, display of which is ceasedcorrespondingly to the opening/closing of the cover, forcibly entersinto the reset period by the reset signal RS inputted from the inputterminal 4 b and the display switching is performed according to “H” and“L” of the selection signal SEL generated thereafter.

The display switching by the selection signal SEL mentioned above isperformed according to the timing of the reset signal RS generatedcorrespondingly to ON/OFF operation of the display switch 11.

As shown in FIG. 1, the one-shot circuit 12 a of the control circuit 12receives the ON/OFF signal of the display switch 11. The OR circuit 12 bgenerates the reset signal RS in response to the reset control signalRSc and an output of the one-shot circuit 12 a. The reset control signalRSc is generated by the timing signal generator circuit 12 c.

Incidentally, the one-shot circuit 12 a is triggered bidirectionally ata rising edge and a falling edge of the ON/OFF signal of the displayswitch 11 and generates a one-shot pulse P, which is “H”, for a constanttime period when the display switching switch 11 is turned ON to OFF orOFF to ON. The constant time period is set to the usual reset period RTor longer.

The output of the one-shot circuit 12 a is sent to the selection signalgenerator circuit 12 d as the reset signal RS. The selection signalgenerator circuit 12 d is a latch circuit constructed with a flip-flop,which generates the selection signal SEL by latching the ON/OFF signalof the display switching switch 11 as “H” or “1” and “L” or “0”correspondingly to the rising signal at the output of the one-shotcircuit 12 a. Thus, the display switching is performed at a time when anorganic EL panel on the non-display side which is terminated, is resetor thereafter.

FIG. 2 is a timing chart when the display switching is performed.

FIG. 2(a) shows the ON/OFF signal (display switching signal) of thedisplay switch 11, FIG. 2(b) shows a display start pulse DSTP, FIG. 2(c)shows the reset control signal RSc, FIG. 2(d) shows a peak generatingpulse Pp and FIG. 2(e) shows a terminal pin drive current in which asolid line shows a drive current and a dotted line shows a drivevoltage.

In FIG. 2(a), the display switch 11 is OFF and the cover of, forexample, the portable telephone set is opened, so that the organic ELpanel 2 is in display state.

In the usual display state, the reset signal RS is generated by the ORcircuit 12 b upon the reset control signal RSc and the reset period RTof the organic EL panel in the display state is ended at the rising edgeof the display start pulse DSTP. At this time point, the reset signal RSfalls and the display period D is started. Then, the peak generationpulse Pp is generated, which is counted by a counter in a constant timeperiod from the start of the display period D. At a count end time, thereset signal RS rises again and the operation enters into the resetperiod RT. As a result, a pin drive current shown in FIG. 2(e) isgenerated.

When the cover is closed and the display switch 11 is turned from OFF toON as shown in FIG. 2(a), the display drive is switched from the maindisplay panel to the sub-display panel. In the display period D shown inFIG. 2(a), in response to the ON/OFF signal (display switching signal)from the display switch 11, the one-shot circuit 12 a generates aone-shot pulse P shown in FIG. 2(f) as the reset signal RS shown in FIG.2(g). Therefore, the organic EL panel in the display state is forced toenter into the reset period RT by the one-shot pulse.

As a result, the analog switches 44X are turned ON to set the outputpins 5 to the reset voltage VR. Incidentally, since the cathodes of theOEL elements are grounded in the display period D, the terminal voltagesof the OEL elements 7 are reset by the turning ON of the analog switches44X.

At a time when the reset period RT corresponding to the one-shot pulse Pis ended, the selection signal SEL is generated by the selection signalgenerator circuit 12 d as shown in FIG. 2(h). Since the display switch11 is turned ON within the display period D as shown in FIG. 2(a), theselection signal SEL becomes “H” at the end of the reset period.

Further, the scan circuit 41 corresponding to the organic EL panel 2,which becomes non-display state by the selection signal SEL “H”,receives the selection signal SEL “L” as the enable signal, so that thetwo switches of the switch circuit, which is constructed with the Pchannel MOS transistor and the N channel MOS transistor, of each CMOSoutput circuit 6 connected to the cathode connection line Y (Y1, Y2, . .. Yi . . . ) of the row side scan circuit shown in FIG. 3 are turned OFFto thereby set the output terminals of the CMOS output circuits 6 tohigh-impedance (Hi-Z).

The above description is in the case where the display switch 11 isturned ON during the display period D. When the display switch 11 isturned OFF during the display period D, the selection signal SEL becomes“L” and the relation between the organic EL panels 2 and 3 is reversed.When the period of the one-shot pulse P overlaps with the reset periodRT or the switching of the display switch 11 is done within the resetperiod RT, the reset period of the reset control signal RSc overlaps theone-shot pulse P. Therefore, the reset period is unchanged or becomesslightly longer by the overlapping period of the reset period of thereset control signal RSc and the one-shot pulse P and the displayswitching operation is the same as described above.

That is, when the display switching switch 11 becomes OFF, the selectionsignal SEL “L” is generated at the end of the reset period or after thereset period is ended by the one-shot pulse P and, when the displayswitching switch 11 becomes ON, the selection signal SEL becomes “H”after the reset period is ended by the one-shot pulse P. The selectionsignal SEL thus generated is sent to the respective row side scancircuits 41 and 42, so that these circuits perform the scanning,selectively.

Therefore, when the display panel whose display is to be stopped,receives the ON/OFF signal (display switching signal) from the displayswitch 11 in the display period, the display switching from one displaypanel to the other display panel is performed after the reset signal RSis generated according to the one-shot pulse P upon the ON/OFF signaland the one display panel is reset. The other display panel whosedisplay is to be started is started upon the display start pulse DSTP ina next reset period RT, as shown in FIG. 2(i).

When the ON/OFF signal of the display switch 11 is generatedcorrespondingly to the reset period RT of the reset control signal RSc,the display panel whose display is to be started by the displayswitching is started in a reset period RT after the next reset period RTshown in FIG. 2(i) by blocking the display pulse DSTP after this resetperiod RT.

Incidentally, the overlapping of the reset period of the control signalRSc and the period of the one-shot pulse P can be detected when theselection signal SEL is changed in the reset period RT of the resetcontrol signal RSc, that is, whether or not there is a rising or fallingof the selection signal SEL in the reset period RT of the reset controlsignal RSc.

When the cover of, for example, the portable telephone set housing theorganic EL display device 1 is closed, the operation of the organic ELdisplay device enters into the reset period RT forcibly and the scanoperation of the row scan circuit 41 of the organic EL panel 2 isstopped and the scan operation of the row scan circuit 42 of the organicEL panel 3 is started at the start of the reset period RT. When thecover of the portable telephone set is opened, the operation of theorganic EL display device enters into the reset period RT forcibly andthe scan operation of the row scan circuit 42 of the organic EL panel 3is stopped and the scan operation of the row scan circuit 41 of theorganic EL panel 2 is started at the start of the reset period RT.

Further, the display panel whose display is stopped does not emit lighterroneously since all of the cathode connection lines Y become Hi-Zafter the resetting.

Incidentally, in the portable telephone set, in order to emphasize adisplay of telephone number, etc., on a display screen thereof, adisplay area is defined in a center portion of the display screen and anarea surrounding the display area is made black or single colored area.Alternatively, there is a zebra color display formed by alternatingblack lines and white lines. However, as described previously, when thenumber of OEL elements connected to one column driver becomes 5,000 ormore, drive current flows to a certain column line from other columnline through parasitic capacitance of the OEL elements connected to thecolumn line of the non-lighting display panel and the amount of thecurrent becomes large. Due to this fact, the display luminance of thecolumn lines displaying black level is pushed up to gray level.

FIG. 4 shows an embodiment of the present invention, for preventing sucherroneous emission.

In FIG. 4, the OEL elements 7 of the non-lighting organic EL displaypanel 3 are shown by capacitors Cp, respectively. The capacitors Cp areparasitic capacitances of the OEL elements 7.

In the case where the organic EL panels 2 and 3 are driven by the commondriver 4 and, for example, the display panel 2 is lighting, theparasitic capacitances Cp of the OEL elements 7 of the non-lighting sidedisplay panel 3 are connected in parallel to the column line Xi.

In this embodiment, diodes Da, Db, Dc, . . . Dn are inserted in forwarddirection of the drive current between the column lines Xa, . . . Xi, .. . Xn of the organic EL panels 2 and 3 and the terminal pins of theorganic EL panels 2 and 3 connected to these column lines, respectively,as shown in FIG. 4.

Further, a row line Yo for connecting the cathodes of the diodes Da toDn together is provided and diodes Dsa, Dsb, Dsc, . . . Dsn are providedin the forward direction between the row line Yo and the cathode sidesof the diodes Da to Dn so that the row line Yo is connected to thecathodes of the respective diodes Da to Dn. CMOS output circuits 6 a andbuffer amplifiers (voltage-followers) 6 b are provided in the row sidescan circuits 41 and 42, respectively, to connect outputs of the bufferamplifiers 6 b to the row lines Yo of the organic EL panels 2 and 3.Inputs of the buffer amplifiers 6 b are connected to the outputs of theCMOS output circuits 6 a, so that the buffer amplifies 6 b receive theoutput voltages of the CMOS output circuits 6 a.

Each of the CMOS output circuits 6 a includes a switch (P channel MOStransistor) SW1 connected to the power source line +Vcc and a switch (Nchannel MOS transistor) SW2 connected to a constant voltage source 6 c.The voltage of the constant voltage source 6 c is Vs.

The diodes Da to Dn inserted in the respective column lines formsindirect drive current preventing circuits for blocking the indirectdrive currents and reducing parasitic capacitances of indirect paths,respectively. Further, the diodes Dsa to Dsn, the row lines Yo, the CMOSoutput circuits 6 a and the buffer amplifiers 6 b form dischargecircuits for blocking reverse flow of the indirect drive current of thecolumn lines Xa to Xn.

Incidentally, the diodes Da to Dn and Dsa to Dsn have PN junctionsformed directly in forming the OEL elements before light emittingmaterial is supplied. These diodes are formed together with the OELelements used for luminance display. Therefore, these diodes do not emitlight even when drive currents flow through them. These diodes may bediode-connected transistors or Shottky diodes having low forward dropvoltage.

The organic EL panel 2, which is lighting side display panel, receives“H” signal obtained by inverting the selection signal SEL “L” (in thestate when the cover is opened) from the selection signal generatorcircuit 12 d by the inverter 43 and the CMOS output circuit 6 a turnsthe switch SW1 ON and the switch SW2 OFF by inverting the “H” signalthus obtained into “L” by an inverter (not shown) provided inside of theCMOS output circuit 6 a.

Therefore, the voltage of the row line Yo is pulled up to the powersource line voltage +Vcc and becomes “H”. As a result, the diodes Dsa toDsn of the organic EL panel 2, which is the lighting side display panel,are reverse-biased and turned OFF. Therefore, the discharge circuits forblocking reverse current flow are disconnected from the column lines andthe diodes Da to Dn are turned ON by the scanning of row side 1horizontal line for the organic EL panel 2 and have no relation to thelighting side display operation.

On the other hand, the organic EL panel 3, which is non-lighting sidedisplay panel, receives the selection signal SEL “L” from the controlcircuit 12 and the CMOS output circuit 6 a turns the switch SW1 OFF andthe switch SW2 ON by inverting the “L” signal thus obtained into “H” bythe inverter (not shown) provided inside of the CMOS output circuit 6 a.Therefore, the row line Yo is set to voltage Vs of the constant voltagesource 6 c through the buffer amplifiers 6 b. The voltage Vs is lowerthan a lowest voltage of the voltages of the respective column lines,which are generated by the drive currents supplied from the column linesfor turning the diodes Da to Dn and Dsa to Dsn ON, by 1.4V (=0.7V×2,where 0.7V is the forward drop voltage of the diode) or more and thevoltage Vs is selected such that current flowing under this conditionbecomes very small. Incidentally, a resistor having large resistancevalue may be inserted in series with the voltage source outputting thevoltage Vs, on demand.

As a result, the diodes Da to Dn and Dsa to Dsn of the non-lighting sideorganic EL panel 3 are forward-biased and turned ON. In this case, theCMOS output circuits 6 turn the switch circuits connected to the outputterminals OFF in response to the selection signal SEL “L” to set thehigh impedance (Hi-Z) as described previously. As a result, all of thenon-lighting side cathode connection lines (Y1, Y2, . . . Yi . . . )except the row line Yo of the scan circuit 42 become Hi-Z.

Since, in this embodiment, the driver IC 4 is commonly used for thedisplay panels, the drive current of the organic EL panel 2, which isthe lighting side display panel, is supplied to the column lines of theorganic EL panel 3, which is the non-lighting side display panel.However, the drive currents flow through the diodes Da to Dn, the diodesDsa to Dsn and the constant voltage source 6 c to ground GND and do notreturn to other column lines. The drive current itself is very small.

As a result, there is no case where a portion of the drive current fromother column line flows indirectly to the column line of the lightingside organic EL panel 2, which is set to black level and does not outputthe drive current.

For the non-lighting side column line Xi, parasitic capacitance Ci of adiode D1 is connected in series with a parallel circuit of a parasiticcapacitance Csi of a diode Dsi and the parasitic capacitance Cp×m of theOEL elements 7, as shown in FIG. 5. Incidentally, m is the number of OELelements 7 connected to the column line Xi.

The parasitic capacitance Ci of each of the diodes Da to Dn is in theorder of several pF and is connected in series to each column line.Therefore, a total parasitic capacitance can be restricted to several pFor smaller regardless of the number of the OEL elements 7.

In a case where a drive current of a certain column line flows to alighting side column line, for example, the column line Xi indirectly, aparasitic capacitance Ci of a diode D1 of the certain column linecoupled through its OEL element is further connected in series to thecircuit having the parasitic capacitance Ci of the diode D1 of thecolumn line Xi shown in FIG. 5. Therefore, the indirect drive current isblocked by the parasitic capacitance Ci of the diode D1 inserted inseries to the column line.

Further, a portion of the drive current of the lighting side column lineis sunk through the constant voltage source 6 c to ground GND. In thiscase, since an amount of the current is determined by the voltage of theconstant voltage source 6 c and is minute, the current does notinfluence on the lighting side drive current substantially.Incidentally, it may be possible to insert a high resistance circuit inseries with the constant voltage source 6 c.

Incidentally, the discharge circuit composed of the diodes Dsa to Dsn,the row line Yo, the CMOS output circuit 6 a and the buffer amplifier 6b may be not always necessary. That is, since the indirect flow of thedrive current can be reduced by only the diodes Da to Dn and, if blacklevel is pushed up to gray level, the gray level is very low, it ispossible to prevent erroneous light emission, which is recognizable byeyes. That is, the discharge circuit may be added on demand.

Assuming that the timing control signal divides the display periodcorresponding to the scan period of one horizontal line from the resetperiod (scan switching period in vertical direction) corresponding tothe retrace period, it is usual in the drive of the passive matrix typeorganic EL panel to use the timing control signal as the reset controlsignal. The reset signal RS is usually used in a portion of the resetperiod RT corresponding to the retrace period.

Therefore, the switching between the organic EL panel 2 and the organicEL panel 3 and the start of operation may be performed according to notthe reset control signal but the timing control signal. They may beperformed according to the reset signal RS. In the latter case, it ispossible to start the operations at the start of the reset periodcorresponding to the retrace period.

In the described embodiment, although the operation of the vertical scancircuit corresponding to the display panel whose display is to bestopped is stopped by stopping the scan operation, it is of coursepossible to directly stop the operation of the vertical scan circuit.

Further, in the described embodiment, the display switch is turned ONwhen the cover of the portable telephone set including the displaydevice 1 is closed. However, the switch may be turned OFF when the coveris closed. In such case, the levels “H” and “L” of the selection signalare reversed.

Incidentally, since it is possible to easily reverse the logic signalsby such as an inverter, it is possible to perform the selectionoperation even when the logic signals are reversed. Further, the displayswitch is not limited to such as a push button. For example, an opticalsensor capable of generating a detection signal in response to lightwhen the cover is opened may be used as the display switch. Othersensors for detecting the display switching may be used. Therefore, theswitch or the switch circuit may include a sensor.

Further, in the described embodiment, the main display panel (organic ELpanel 2) and the sub display panel (organic EL panel 3) have the columnpins connected to the respective output pins of the driver IC 4.However, it may be possible to provide a driver IC for the main displaypanel.

Further, in the described embodiment, the operation of the vertical scancircuit of one of the first and second organic EL panels, which isdriven (or displays) according to the selection signal, is preferablystarted after the operation of the vertical scan circuit of the otherorganic EL panel is stopped. The operation stoppage in this case is notlimited to a temporary stoppage of the scan operation or to the standbystate. The operation of this circuit itself may be stopped.

INDUSTRIAL APPLICABILITY

As described hereinbefore, the constant voltage resetting is performedsuch that the preset voltage of the output pins of one horizontal line,which is driven in the display period, becomes Vz. However, the resetvoltage may be ground potential or other reference potential. Further,although the two passive matrix type organic EL panels are described,the present invention can be applied to active matrix type organic ELpanels, which can be reset by the output pins. In such case, the columnpins are replaced by a data line, pixel circuits are arranged instead ofthe OEL elements and OEL elements of the pixel circuits are driventhrough capacitors provided in the pixel circuit for storing drivecurrent. Further, although the MOSFETs are used mainly, it is of coursepossible to use bipolar transistors instead of the MOSFETs. Further, theN channel (or npn) type transistors may be replaced by P channel (orpnp) transistors and the P channel transistors may be replaced by Nchannel (or npn) type transistors. In such case, the power sourcevoltage is usually negative and the transistor provided in the upstreamside is provided in the downstream side.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block circuit diagram according to an embodiment of anorganic EL display device in which a current drive circuit is usedcommonly for two passive matrix type organic EL panels.

FIG. 2 is a timing chart when a display is switched.

FIG. 3 is a circuit diagram for explaining an organic EL panel whosedisplay is stopped when a display switching is performed in its row sidescan circuit.

FIG. 4 is a circuit diagram for preventing erroneous emission in alighting side display panel when display luminance of column linesdisplaying black level becomes gray level.

FIG. 5 is a circuit diagram for explaining a load impedance of anon-lighting side column line in FIG. 4.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   -   1 . . . organic EL display device    -   2,3 . . . passive matrix type organic EL panel    -   4 . . . driver IC    -   6 . . . CMOS output circuit    -   5,5 a,5 i,5 n . . . output pin    -   6,6 a . . . CMOS output circuit    -   6 b . . . buffer amplifier    -   6 c . . . constant voltage source    -   7 . . . OEL element    -   40,40 a˜40 n . . . output stage current source    -   11 . . . display switch    -   12 . . . control circuit    -   12 a . . . one-shot circuit    -   12 b . . . OR circuit    -   12 c . . . timing signal generator circuit    -   12 d . . . selection signal generator circuit    -   41,42 . . . row side scan circuit    -   43 . . . inverter    -   44 . . . reset circuit    -   46 . . . D/A converter circuit    -   44 a,44 i,44 n,44 x . . . analog switch    -   45 . . . current mirror circuit    -   Da,Di,Dn . . . diode    -   Y0,Y1,Y2,Yi . . . cathode connection line

1. An organic EL display device, which includes a first and secondorganic EL display panels, selects either one of the organic EL displaypanels according to a selection signal and drives the selected organicEL display panel to perform a predetermined display, comprising aplurality of current drive circuits having output pins connectedcommonly to data lines or column pins of the first and second organic ELpanels, for outputting drive currents for driving organic EL displayelements from the output pins to the data lines or the column pinsconnected to the output pins, a reset circuit connected to the outputpins for resetting terminal voltages of the organic EL display elementsto a predetermined voltage in a reset period, and a first and secondscan circuits provided correspondingly to the first and second organicEL display panels for scanning scan lines in a row direction or avertical direction of the first and second organic EL panels, whereinone of the first and second scan circuits corresponding to one of theorganic EL panels, which is to be driven, is operated in the resetperiod according to the selection signal and an operation of the otherscan circuit is stopped or the scan operation of the other scan circuititself is stopped for driving said one organic EL panel and ceasing theother organic EL panel.
 2. The organic EL display device as claimed inclaim 1, wherein the reset period is determined by a timing controlsignal or a reset control signal for dividing the display periodcorresponding to the scan period of one horizontal line from the resetperiod corresponding to a retrace period of horizontal scan, the resetcircuit performs a reset operation according to one of a signalcorresponding to the timing control signal, the reset control signal anda reset signal in the reset period.
 3. The organic EL display device asclaimed in claim 2, wherein the first and second organic EL displaypanels are of the passive matrix type, the output pins are connected tothe column pins of the first and second organic EL display panels,respectively, and a start of scan operation of the scan circuit for oneof the first and second organic EL display panels is after a scanoperation of the other scan circuit is stopped or the scan operation ofthe other scan circuit itself is stopped.
 4. The organic EL displaydevice as claimed in claim 3, wherein the stopping of the scan operationof the scan circuit of the other organic EL display panel or thestopping of the scan operation itself is performed after the terminalvoltage of the organic EL display element is reset by the reset circuitand all of the output terminals of the scan circuit of the other organicEL display panel, to which the scan lines of the other organic ELdisplay panel are connected, are set to high impedance.
 5. The organicEL display device as claimed in claim 4, further comprising a switch,which performs an ON/OFF operation according to an open/close operationof a cover of a device having the organic EL display device, wherein oneof the first and second organic EL display panels is a main displaypanel and the other organic EL display panel is a sub-display panel andthe selection signal is generated correspondingly to ON/OFF of theswitch.
 6. The organic EL display device as claimed in claim 5, whereinthe scan line corresponds to one horizontal line and the switch performsON/OFF operation correspondingly to a signal from an optical sensorprovided in the device.
 7. The organic EL display device as claimed inclaim 1, wherein the first and second organic EL display panels are ofthe passive matrix type, the output pins are connected to column pins ofthe first and second organic EL display panels, respectively, and firstdiodes for preventing reverse current flow are provided between a columnline connected to the column pins of the first and second organic ELdisplay panels and the column pins, respectively.
 8. The organic ELdisplay device as claimed in claim 7, wherein the first and secondorganic EL display panels include connection lines in row direction forconnecting cathodes of the first diodes, respectively, second diodesinserted between the cathode sides of the first diodes and theconnection lines in forward direction corresponding to the first diodesare provided, the connection lines of the first and second organic ELdisplay panels are selectively connected to a first potential line or asecond potential line, the first potential is to reverse biasing thesecond diodes and the second potential is to forward biasing the seconddiodes.
 9. The organic EL display device as claimed in claim 8, whereinthe connection lines of the first and second organic EL display panelsare connected to the first or second potential line through bufferamplifiers, respectively, the second diodes of the one organic ELdisplay panel are connected to the first potential line of the oneorganic EL display panel, the second diodes of the other organic ELdisplay panel are connected to the second potential line of the otherorganic EL display panel and all of the output terminals of the scancircuit of the other organic EL display panel, to which the scan linesof the other organic EL display panel are connected, are set to highimpedance.
 10. The organic EL display device as claimed in claim 9,wherein voltages of the first potential line and the second potentialline are lower than a voltage, which is the lowest voltage of therespective column pins generated by the drive currents supplied to therespective column pins reduced by a sum of the forward drop voltage ofthe first diodes and the forward drop voltage of the second diodes. 11.The organic EL display device as claimed in claim 9, wherein the scanoperation of the scan circuit of one of the first and second organic ELdisplay panels is started after the scan operation of the other scancircuit is stopped or the operation of the other scan circuit itself isstopped.
 12. The organic EL display device as claimed in claim 11,wherein the stoppage of the scan operation of the other scan circuit orthe stoppage of the scan operation of the other scan circuit itself isperformed after the resetting of the terminal voltage of the organic ELdisplay elements by the reset circuit and all of the output terminals ofthe scan circuit of the other organic EL display panel, to which thescan lines of the other organic EL display panel are connected, are setto high impedance.